两种解法.一.x+2y=4/5,x-y=25/4,则（x+2y）*（x-y）= x²+xy-2y² =4/5 * 25/4=5；x²=xy-2y²可化为x²-（xy-2y²）=0解方程组x²+xy-2y² =5和x²-（xy-2y²）=0.两式相...